This application is related to a co-pending application entitled "Detector for a Chromatograph" filed together herewith.
The invention relates to detecting a parameter of a component of a gas in a chromatograph, and more particularly to detecting a signal level that is indicative of the parameter.
Electrical signals are often used to carry parameters relating to many processes. Several aspects of a signal are used to represent the values of the parameters; an important one of these is the highest level which a signal attains during a particular interval of time. In gas chromatograph for example, in which a gas containing molecules of different types is separated into its component parts, a signal is generated whose level is proportional to the number of molecules that pass a sensor.
The chromatograph uses a resistive medium, such as small pieces of firebrick coated with a heavy oil to separate the gas components. A standard-sized sample of gas is carried through the medium at a constant rate by a gas such as helium. The resistive medium resists the passage of the sample gas molecules in proportion to their size. As a result the molecules of a particular size tend to bunch together and exit from the resistive medium about the same time. The smaller molecules exit first, followed by groups of molecules of progressively larger sizes. Each group of a particular size requires a known amount of time, called the elution time, to pass through the resistive medium. Each group is distributed over a small interval of time usually in a normal or Gaussian distribution. Since the shape of the distribution is known, and if the Gaussian peaks are symmetrical, the number of molecules in a group can be determined from the peak value of the distribution.
The passage of the molecules from the resistive-medium is often sensed by a thermistor. The passage of the gas molecules over the thermistor alters the temperature of the thermistor, which in turn alters its resistance. The varying resistance is used to modulate an electrical signal. The shape of the signal will thus be related to the distribution of the molecules in a particular group separated by the resistive medium. The peak value of the signal can then be determined by means of circuitry disclosed in the co-pending application cited above.
Due to gas currents and ambient temperatures in the chromatograph, the sensor often produces an offset signal when there is no sample gas in the chromotograph. If it is not corrected, the offset signal will introduce error into subsequent measurement of the gas components. The zeroing procedure has often been performed by means of a signal divider circuit using a potentiometer. The potentiometer was manually adjusted until the necessary compensating signal was introduced into the circuit to eliminate the offset signal. A manual zeroing process is obviously inconvenient. It is also inadequate in situations where the offset signal can vary quite rapidly as in a chromatograph. An automatic zeroing circuit would thus be a valuable addition to a gas chromatograph.
It is therefore an object of the invention to provide a new and improved zeroing circuit that overcomes these and other disadvantages of the prior art. Other objects will become apparent as the invention is described in detail.